1. Field of the Invention
The present invention relates to a thermal transfer-receiving sheet to be used in combination with a thermal transfer sheet. More specifically, the present invention relates to a thermal transfer-receiving sheet comprising a plain paper on which a receptor layer is formed by using a powdery composition and also to a method for manufacturing the thermal transfer-receiving sheet.
2. Description of the Related Art
Heretofore, various types of thermal transfer recording methods have been known. As one of them, there is known a sublimation type transfer recording method wherein a sublimation dye is used as a coloring material so that an image is obtained by transferring the sublimation dye to a thermal transfer image receiving sheet by use of a thermal head which generates heat in response to recorded signals. Recently, the sublimation type transfer recording is utilized as an image forming means in various fields. Since the sublimation dye is used as a coloring material, the gradation of a printing density can be controlled at will to reproduce a full color image in accordance with the original image in the sublimation type transfer recording.
Furthermore, since the image formed of the dye is very clear and excellent in transparency, the reproduction of intermediate colors and the reproduction of gradation in the image are excellent, thus enabling to form a high-quality image comparable to a silver salt-based photographic image.
As for the thermal transfer-receiving sheet for use in thermal transfer recording methods, there is known a thermal transfer-receiving sheet which comprises a plastic sheet or a synthetic paper as a substrate whose one side or both sides are provided with a dye receptor layer comprising a dyeable resin.
Also proposed is a thermal transfer-receiving sheet which comprises a plain paper as a substrate. The image formed on the thermal transfer-receiving sheet which uses a plain paper as a substrate is comparable to a printed product obtained by an ordinary printing method in terms of feel such as surface gloss and thickness. Further, contrary to the thermal transfer-receiving sheet using the plastic sheet or the synthetic paper as a substrate, the thermal transfer-receiving sheet using the plain paper as the substrate is advantageous in, for example, that it can be bent and that bookbinding or filing of even a stack of several sheets of it is possible. Furthermore, since the plain paper is cheaper than the synthetic film or sheet, the thermal transfer-receiving sheet using the plain paper can be manufactured at a lower cost.
In the case of the thermal transfer-receiving sheet using the plain paper as the substrate, in order to obtain a high-quality image, it is necessary to solve problems such as minute irregularity of the surface and lack of cushioning property. Some methods have been proposed to solve these problems.
According to one method proposed, in order to supplement the cushioning property there is disposed a foam layer, which comprises a thermally decomposing foaming agent, foamable microcapsules, and the like, between the substrate (plain paper) and the receptor layer. This method, however, is associated with problems, for example, that the feel of the thermal transfer-receiving sheet is limited to a mat; that the manufacturing process is complicated and the manufacturing cost is high; and that a protective layer is necessary to protect the foam layer from a coating liquid which forms the receptor layer.
According to another method proposed, in order to supplement the cushioning property there is disposed a thermal insulation layer, which comprises fine resin particles, between the substrate (plain paper) and the receptor layer. This method, however, is associated with problems, for example, that the feel of the thermal transfer image receiving sheet is limited to a glossy; and that a protective layer is necessary to protect the thermal insulation layer of resin particles from a coating liquid which forms the receptor layer.
In these methods, the receptor layer is formed by applying a coating liquid to the substrate and thereafter drying the resulting layer. In contrast with these methods, Japanese Patent Application Laid-Open (JP-A) Nos. 8-112,974 and 8-224,970 propose a thermal transfer-receiving sheet comprising a plain paper having on the surface thereof a receptor layer made from a powdery coating composition containing a dyeable resin.
In the technique utilizing the powdery coating composition, a powdery coating composition is first prepared by a process comprising melt-blending a composition composed of a resinous substance, a white pigment, an electrification-controlling agent, an offset-preventing agent, and the like, cooling and pulverizing the melt-blended product, and classifying the resulting powder so that a product having an appropriate mean particle diameter is obtained. The powdery coating composition thus obtained is adhered as a layer to the surface of a sheet of plain paper or the like constituting a substrate by means of an electrostatic powder-coating method or the like, and the powder layer is then heated, pressed, or alternatively heated and pressed to fix the powder layer so that a dye receptor layer is formed. The thermal transfer-receiving sheet prepared in this way is advantageous in, for example, that the manufacturing process and the layer structure are simple and that the feel of a plain paper is not impaired.
When the substrate surface is coated with a powdery coating composition, even after the coated layer of the powdery coating composition is fixed by heating and/or pressing, the voids between powder particles do not perfectly disappear and some of the voids remain as pores. Therefore, the receptor layer formed is not a perfectly compact continuous layer, and minute pores and cracks are undesirably present inside the receptor layer. To the contrary, such undesirable phenomena do not occur if the receptor layer is formed by using a coating liquid. In addition, since the plain paper is a porous substrate, part of the powdery coating composition coated on the plain paper infiltrates into pores of pulp. The infiltration of the powdery composition into the pores of pulp is further promoted by the heating and pressing in the fixing process.
The above-described phenomenon makes it difficult to form a receptor layer having a constant thickness, because, even if a constant amount of the powdery composition is applied on the substrate surface, some pores are formed in the coated layer and part of the powdery composition infiltrates into the pores of pulp. Accordingly, the surface of the receptor layer thus obtained is markedly influenced by the surface irregularity of the plain paper constituting a substrate and tends to have such problems as lack of cushioning property and rough surface. As a result, it was difficult to obtain a printing sensitivity and an image quality of a satisfactory level.
In addition, if a single side of the plain paper constituting the substrate was provided with the receptor layer, the difference in shrinkage between the receptor layer and the substrate induced by heat or moisture led to defects such as curl in a printing process and environmental curl, thus presenting a significant impediment to the practical use of the plain paper as the substrate. Further, since the plain paper was used as the substrate, the heat delivered from the thermal head at the time of image printing caused dimensional change of the substrate to an extent that the image registration in printing sometimes deviated.
Yet another problem was that the scratch resistance of the receptor layer was so poor that it was difficult to write on the receptor layer with a pencil or the like.